Environmentally friendly surface treated steel sheet for electronic parts excellent in soldering wettability and resistance to rusting and formation of whisker

ABSTRACT

The present invention provides a surface treated steel sheet for electronic components which does not include lead, which is a hazardous substance with environmental impact, and, in particular, satisfies the solder wettability after retort treatment, a rust-proof property and a whisker-proof property simultaneously. More specifically, the present invention is a surface treated steel sheet with less environmental impact for electronic components excellent in solder wettability, a rust-proof property and a whisker-proof property and having an Sn—Zn alloy layer which is formed by plating Sn and Zn on a steel sheet or a Ni plated steel sheet and then applying thermal diffusion treatment, or by plating Sn—Zn alloy on a steel sheet or a Ni plated steel sheet, characterized in that the amount of said Sn—Zn alloy layer is not less than 3 g/m 2 , the Zn/Sn ratio (in weight ratio) is 0.01 to 10, more preferably 0.01 to 0.1, and an inorganic film mainly composed of magnesium phosphate is formed in the amount of 1 to 100 mg/m 2  in terms of the amount of P+Mg on said Sn—Zn alloy layer.

TECHNICAL FIELD

[0001] The present invention relates to a surface treated steel sheetfor electronic components, used for electronic components of electricproducts, excellent in solderability, a rust-proof property and awhisker-proof property, and not containing substances with environmentalimpact such as lead, etc.

BACKGROUND ART

[0002] For electronic components of electric products, a tinplate, aterneplate, a solder plated steel sheet or the like, as a surfacetreated steel sheet particularly excellent in solderability, isgenerally used.

[0003] As an example, a surface treated steel sheet having an Sn platedlayer of 8.4 to 11.2 g/m² on each surface of a steel sheet (hereunderreferred to as “#75 to #100 tinplate”) has been used as a surfacetreated steel sheet having such excellent solder wettability as to besoldered by dipping the steel sheet in a molten solder bath for a shortperiod of time. However, a space between electronic components hasnarrowed due to the recent downsizing of electric products, and as aresult, tinplate has produced the problems of direct short circuits, thedestruction of an insulating layer, and the like, caused by acicularsingle crystals (whiskers) grown from a tin plated layer. For thatreason, a terneplate or a solder plated steel sheet which does notgenerate whiskers has mainly been used.

[0004] As methods for preventing the generation of whiskers, advocatedhave so far been the method of applying alloy plating (Japanese ExaminedPatent Publication No. S58-2598, Japanese Unexamined Patent PublicationNo. S49-129, etc.) and the method of applying post-treatment afterplating (Japanese Examined Patent Publication Nos. S56-47955 andS56-47956, Japanese Unexamined Patent Publication Nos. S59-143089 andS62-77481, etc.). However, those methods have scarcely been put topractical use because the alloy plating or the post-treatment hindersthe solderability. In the meantime, there is a method which is put topractical use by the optimization of an alloy composition and a chromatetreated layer as disclosed in Japanese Unexamined Patent PublicationNos. H2-270970 and H3-183796.

[0005] In recent years, the regulations against hazardous substanceswith environmental impact have been enforced in view of globalenvironmental problems and, in particular, hexavalent chromium and leadare objects of regulation. Therefore, in addition to the need for amaterial with which lead-tin solder is substituted, the need for amaterial as a plated steel sheet with which a terneplate or a solderplated steel sheet is substituted is getting increased. In the case ofsmall-sized electronic components in particular, they are soldered bydipping in a molten solder bath for a short period of time so that theyare soldered with high efficiency.

[0006] Further, as the electronic components are sometimes stored for along period of time, the performance of the electronic components afterlong storage is evaluated in the accelerated manner by applying anaccelerating treatment in a retort or the like for experimentallyreproducing the long storage conditions, and a flux is also beingsubstituted with a type of material having a low activity and notcontaining chlorine. For a surface treated steel sheet for electroniccomponents, remarkably excellent solder wettability including solderwettability after the accelerating treatment is required.

[0007] As described above, there is a strong demand to provide a surfacetreated steel sheet with less environmental impact for electroniccomponents excellent in both solder wettability and a whisker-proofproperty.

[0008] The object of the present invention is to provide a surfacetreated steel sheet for electronic components which does not includelead, which is a hazardous substance with environmental impact, and, inparticular, satisfies the solder wettability after retort treatment, arust-proof property and a whisker-proof property, simultaneously.

DISCLOSURE OF THE INVENTION

[0009] The present invention is a surface treated steel sheet capable ofsecuring good a whisker-proof property and a good rust-proof propertywhich have been problems in tinplate, while securing more excellentsolder wettability, after retort treatment, than a terneplate used forapplication to electronic components which are currently soldered bydipping in a molten solder bath for a short period of time.

[0010] Such a surface treated steel sheet can be attained by; in asurface treated steel sheet for electronic components having an Sn—Znalloy layer which is formed by plating Sn and Zn on a steel sheet or aNi plated steel sheet and then applying thermal diffusion treatment, orby plating Sn—Zn alloy on a steel sheet or a Ni plated steel sheet:specifying the amount of Sn—Zn alloy and a Zn/Sn ratio; and applyingthereon an inorganic film mainly composed of magnesium phosphate as thesubstitution of a conventional chromate film.

[0011] That is, the present invention is a surface treated steel sheetwith less environmental impact for electronic components: excellent insolder wettability, a rust-proof property and a whisker-proof property;and having an Sn—Zn alloy layer which is formed by plating Sn and Zn ona steel sheet or a Ni plated steel sheet and then applying thermaldiffusion treatment, or by plating Sn—Zn alloy, characterized in that:the amount of said Sn—Zn alloy layer is not less than 3 g/m²; the Zn/Snratio (in weight ratio) is 0.01 to 10, more preferably 0.01 to 0.1; andan inorganic film mainly composed of magnesium phosphate is formed inthe amount of 1 to 100 mg/m² in terms of the amount of P+Mg on saidSn—Zn alloy layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic drawing showing the surface layer structurein the cross section of the plated layer of a surface treated steelsheet according to the present invention.

[0013]FIG. 2 is a map showing the relation among the P+Mg depositedamount of an inorganic film, the Zn/Sn ratio of an Sn—Zn alloy layer andsolder wettability.

[0014]FIG. 3 is a graph showing the relation between the Zn/Sn ratio ofan Sn—Zn alloy layer and a whisker-proof property.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] The present invention will hereunder be explained in detail.Firstly, the reasons for limiting the scope of a surface treated steelsheet according to the present invention will be explained.

[0016]FIG. 1 schematically shows the surface layer structure in thecross section of the plated layer of a surface treated steel sheetaccording to the present invention. In the figure: numeral 1 designatesan Sn—Zn alloy layer which is formed by plating Sn and Zn on a steelsheet or a Ni plated steel sheet (not shown in the figure) and thenapplying thermal diffusion treatment, or by plating Sn—Zn alloy on asteel sheet or a Ni plated steel sheet; and numeral 2 an inorganic filmmainly composed of magnesium phosphate and formed on an Sn—Zn alloylayer 1, which is most intrinsic to the present invention.

[0017] An Sn—Zn alloy layer is a plated film which constitutes the basisof the present invention, and the deposited amount is required to be atleast 3.0 g/m² or more for securing good solder wettability and a goodrust-proof property. The upper limit thereof is not particularlydetermined in the present invention, but it is preferable that the upperlimit is about 50 g/m² from the viewpoint of the cost.

[0018] With regard to methods for forming an Sn—Zn alloy layer, inaddition to the method of forming the alloy layer by electroplating Snand Zn on a steel sheet or a Ni plated steel sheet and then applyingthermal diffusion treatment to the steel sheet or the Ni plated steelsheet, there are a method of directly coating a steel sheet with Sn—Znalloy using electroplating and a method of dipping a steel sheet in amolten Sn—Zn alloy bath, namely the hot dipping method. Further, byusing an Ni plated substrate steel sheet instead of said steel sheet inthe above three methods as disclosed in Japanese Unexamined PatentPublication Nos. H2-270970 and H3-183796, it is possible to form an Niplated layer or an Fe—Ni diffusion layer at each interface on a steelsheet and an Sn—Zn alloy layer on the surface layer. Note that in thepresent invention an Sn—Zn alloy layer is formed on both surfaces of asteel sheet or a Ni plated steel sheet. Accordingly, the presentinvention does not particularly restrict a method for forming an Sn—Znalloy layer.

[0019] By forming an Ni plated substrate layer, when an Sn—Zn alloylayer is thin, the Sn—Zn alloy layer is uniformed and the rust-proofproperty improves.

[0020] Next, the restrictions related to the Zn/Sn ratio (in weightratio) of an Sn—Zn alloy layer and an antioxidation film will beexplained. By forming a thin inorganic film, which is mainly composed ofmagnesium phosphate in the amount of 1 to 100 mg/m² in terms of theamount of P+Mg, on an Sn—Zn alloy layer having a deposited amount of notless than 3 g/m² and a Zn/Sn ratio (in weight ratio) of not more than10.0 by dipping in a bath mainly composed of magnesium biphosphatesolution and then drying, it becomes possible to suppress the growth ofthe oxide film after accelerating treatment such as retort treatment andto secure excellent solder wettability and a rust-proof property of thesame level as that of a conventional chromate film, as seen in FIG. 2.

[0021] Further, by setting the lower limit of a Zn/Sn ratio (in weightratio) at not less than 0.01, it becomes possible to secure a goodwhisker-proof property as shown in FIG. 3.

[0022]FIG. 2 shows the relationships between the deposited amount ofP+Mg which represents the amount of an inorganic film mainly composed ofmagnesium phosphate and formed on a surface layer, the Zn/Sn ratio (inweight ratio) of an Sn—Zn alloy layer, and solder wettability, when anSn—Zn alloy amount is 5.0 to 20.0 g/m². In this case, the solderwettability was evaluated by using a device to record the deteriorationof a solder meniscus with age, employing Sn—Ag lead-free solder as thesolder and two kinds of flux, inactive type and active type, as theflux, and measuring the wettability after test pieces were subjected tothe accelerating treatment in a retort at 105° C. for 8 hours.

[0023] The results of the evaluation are expressed by the mark ⊚ whenthe wet time (zero cross time) is within 3 seconds under the inactiveflux, ◯ when the same is within 3 seconds under the active flux, Δ whenthe same is 3 to 5 seconds under the active flux and × when the same isnot less than 5 seconds under the active flux.

[0024] As is shown in FIG. 2, the range of the Zn/Sn ratio where thesolder wettability is good is not more than 10.0 and, in particular,very good wettability is obtained in the range not more than 0.1 evenunder the inactive flux. with regard to the P+Mg deposited amount, inthe case that the deposited amount is zero, the evaluation result is Δwhen the Zn—Sn ratio (in weight ratio) is in the range of 0.01 to 0.1and × when the same is not less than 1, and therefore, in order for theinorganic film to function as an antioxidation film, the P+Mg depositedamount must be not less than 1 mg/m². With regard to the upper limit ofthe P+Mg deposited amount, though it varies depending on the type of acoating method, there is a tendency to hinder the solder wettabilitywhen the deposited amount exceeds 100 mg/m². Therefore, it is preferablethat the P+Mg deposited amount is not more than 100 mg/m².

[0025]FIG. 3 shows the result on the relation between the Zn/Sn ratio(in weight ratio) of an Sn—Zn alloy layer and the whisker-proofproperty. The test for evaluating the whisker-proof property was carriedout by subjecting the test pieces to 90 degree bending and bulgingprocessing, and then aging for 3 months in the atmosphere of 60° C. intemperature and 90% in relative humidity which was similar to theatmosphere employed in the moisture resistance test. The evaluationitself was carried out by visual inspection and scanning electronmicroscope, and the results of the evaluation are expressed by the mark◯ when the occurrence of whiskers is less than 50 μm and × when the sameis not less than 50 μm.

[0026] As is shown in FIG. 3, with regard to the whisker-proof property,the occurrence of whiskers is less than 50 μm when the Zn/Sn ratio (inweight ratio) is not less than 0.01. From the above explanation, therange of the Zn/Sn ratio (in weight ratio) is limited to not less than0.01 from the viewpoint of the whisker-proof property, not more than10.0 from the viewpoint of the solder wettability under the active flux,and not more than 0.1 from the viewpoint of the solder wettability underthe inactive flux.

EXAMPLE

[0027] The present invention will further be explained based on theexamples hereunder. The evaluation results of the performances in theexamples wherein detailed conditions are varied and the comparativeexamples are summarized in Table 1.

Example 1

[0028] Low carbon cold rolled steel sheets produced by cold rolling andannealing in a usual manner were subjected to degreasing and pickling,by a usual method, and then coated, in order, with Ni plating under thetreatment conditions shown in the item (1), Sn plating under thetreatment conditions shown in the item (2) and Zn plating under thetreatment conditions shown in the item (3). In succession, the steelsheets were subjected to a heat treatment at a steel sheet surfacetemperature of 250 to 350° C. for not less than 0.5 second in theatmosphere using an electric resistance heating method, and the platedfilms mainly composed of Sn—Zn binary alloy were formed thereon.Further, the steel sheets were subjected to post-treatment under theconditions shown in the items (4) and (5) and, after that, to variouskinds of evaluation tests. (1) Ni plating {circle over (1)} Bathcondition NiSO₄.7H₂O: 200-300 g/L (liter) H₂SO₄: 0-50 g/L (liter) H₃BO₃:40 g/L (liter) {circle over (2)} Plating condition Bath temperature:40-50° C. Current density: 5-30 A/dm² (2) Sn plating {circle over (1)}Bath condition Tin sulfate: 20-30 g/L (liter) Phenolsulfonic acid: 20-30g/L (liter) Ethoxylation α-naphtholsulfonic acid: 2-3 g/L (liter){circle over (2)} Plating condition Bath temperature: 35-45° C. Currentdensity: 2-30 A/dm² (3) Zn plating {circle over (1)} Bath conditionZnSO₄.7H₂O: 200-400 g/L (liter) Na₂SO₄: 50-150 g/L (liter) {circle over(2)} Plating condition Bath temperature: 40-50° C. Current density: 5-30A/dm2 (4) Treatment for removing surface layer oxide film Dipping time:3 seconds {circle over (1)} Bath condition H₂SO₄: 10-20 g/L (liter){circle over (2)} Bath temperature Ordinary temperature (20-30° C.) (5)Antioxidation film treatment {circle over (1)} Bath condition magnesiumbiphosphate solution: 1-20 g/L (liter) {circle over (2)} Treatment Bath:Ordinary temperature condition to 50° C. (dip for 3-5 seconds)

Example 2

[0029] Low carbon cold rolled steel sheets produced by cold rolling andannealing, in a usual manner, were subjected to degreasing and picklingby a usual method, and then, in order, coated with Ni plating under thetreatment conditions shown in the item (1) of Example 1, and Sn—Zn alloyplating under the treatment conditions shown in the item (6) below andsubjected to the treatment for removing surface layer oxide films underthe treatment conditions shown in the item (4) of Example 1. Insuccession, the steel sheets were subjected to an antioxidation filmtreatment under the conditions shown in the item (5) of Example 1 and,after that, to various kinds of evaluation tests. (6) Sn—Zn alloy hotdip plating {circle over (1)} Bath condition Sn—Zn alloy {circle over(2)} Plating conditions Bath temperature: 250-300° C. Dipping time: 1second Plating amount: 30-40 g/m² (wiping control)

Example 3

[0030] Low carbon cold rolled steel sheets produced by cold rolling andannealing in a usual manner were subjected to degreasing and pickling bya usual method, and then, in order, coated with Ni plating under thetreatment conditions shown in the item (1) of Example 1, and Sn—Zn alloyplating under the treatment conditions shown in the item (7) below andsubjected to the treatment for removing surface layer oxide films underthe treatment conditions shown in the item (4) of Example 1. Insuccession, the steel sheets were subjected to an antioxidation filmtreatment under the conditions shown in the item (5) of Example 1 and,after that, to various kinds of evaluation tests. (7) Sn—Zn alloyelectroplating {circle over (1)} Bath conditions Alkanolsulfonic acid:10-200 g/L (liter) Bivalent zinc: 1-50 g/L (liter) Bivalent Tin: 100-500g/L (liter) {circle over (2)} Plating conditions Bath temperature:50-60° C. Current density: 10-200 A/dm²

Comparative Example 1-1

[0031] Comparative Example 1-1 was prepared by applying a chromatetreatment under the conditions shown in the item (8) below instead ofthe treatments shown in the items (4) and (5) of Example 1, with theother conditions being the same as Example 1. (8) Chromate treatment{circle over (1)} Bath condition CrO₃: 50-100 g/L (liter) {circle over(2)} Bath temperature: 40-50° C. (dip for 5 seconds)

Comparative Example 1-2

[0032] Comparative Example 1-2 was prepared by eliminating the chromatetreatment shown in the item (8) from Comparative Example 1-1, with theother conditions being the same as Example 1.

Comparative Example 2

[0033] Comparative Example 2 is an electroplated tinplate having the Snplating amount of 11.2 g/m² per each surface (referred to as “#100tinplate”), and the plate is subjected to various kinds of evaluationtests.

Comparative Example 3

[0034] Comparative Example 3 is a lead plated steel sheet (referred toas “terneplate”) having the Pb plating amount of 30 g/m² per eachsurface, and the plate is subjected to various kinds of evaluationtests.

[0035] The above-mentioned Examples according to the present inventionand Comparative Examples were subjected to the evaluation tests shown inthe items (a) to (c) below and their properties were evaluated. Itshould be noted that, with regard to Examples, Sn—Zn alloy platingamount (g/m²), Zn/Sn ratio (in weight ratio) and the P+Mg depositedamount were measured by the methods shown in the items (1) to (3) belowbefore they were subjected to the evaluation.

[0036] (a) Solder Wettability Test

[0037] The solder wettability test was carried out by using a device torecord the deterioration of a solder meniscus with age (SWET-2100,manufactured by Tarutin Kester, Co., Ltd.), employing Sn—Ag—Bi lead-freesolder (SA2515, manufactured by Tarutin Kester, Co., Ltd.) as the solderand two kinds of flux, non-chloride flux (NA200, manufactured by TamuraGiken Co., Ltd.) and active flux containing chlorine (NS828,manufactured by Nihon Superior Co., Ltd.) as the flux, and wettabilitywas measured after the test pieces were subjected to the acceleratingtreatment in a retort at 105° C. for 8 hours.

[0038] The results of the evaluation are expressed by the mark ⊚ whenthe wet time (zero cross time) is within 3 seconds under the inactiveflux, ◯ when the same is within 3 seconds under the active flux, Δ whenthe same is 3 to 5 seconds under the active flux and × when the same isnot less than 5 seconds under the active flux.

[0039] (b) Whisker-Proof Test

[0040] The whisker-proof test was carried out by subjecting the testpieces to 90 degree bending and bulging processing, and then aging for 3months in the atmosphere of 60° C. in temperature and 90% in relativehumidity which was similar to the atmosphere employed in the moistureresistance test. The evaluation itself was carried out by visualinspection and by a scanning electron microscope, and the results of theevaluation are expressed by the mark ◯ when the occurrence of whiskersis less than 50 μm and × when the same is not less than 50 μm.

[0041] (c) Rust-Proof Test

[0042] The rust-proof test was carried out by subjecting the test piecesto the 72 hour continuous salt spraying test specified in JIS (JapaneseIndustrial Standards) Z 2371, and measuring the area ratio of generatedred rust in terms of percentage.

[0043] (1) Measurement of Sn—Zn Alloy Amount (g/m²)

[0044] Each weight of Sn and Zn was determined by applying each value ofSn and Zn measured by a fluorescent X-ray spectrometer to thecalibration curve of each weight of Sn and Zn prepared beforehand, andthen the Sn—Zn alloy amount was obtained by summing each weight.

[0045] (2) Measurement of Zn/Sn Ratio (in Weight Ratio)

[0046] The Zn/Sn ratio was calculated from each weight of Sn and Znobtained by the same method as shown in the item (1).

[0047] (3) Measurement of P+Mg Deposited Amount

[0048] The weight of P was determined by applying the value of Pmeasured by a fluorescent X-ray spectrometer to the calibration curve ofthe weight of P prepared beforehand, the weight of Mg was determined byapplying the value of Mg measured in a solution wherein a coating filmis dissolved by acid using a high frequency inductively coupled plasmaemission spectrometer to the calibration curve of the weight of Mgprepared beforehand, and then the P+Mg deposited amount was obtained bysumming each weight of P and Mg.

[0049] Table 1 shows the details and the evaluation results of theperformances of Examples and Comparative Examples in a summarizedmanner.

[0050] The evaluation results of the performances of Examples are shownin Examples 1-1 to 1-4 which an Sn—Zn alloy layer is formed by applyingthe thermal diffusion alloying treatment after electroplating of Sn andZn, in Examples 2-1 and 2-2 which an Sn—Zn alloy layer is formed byapplying hot dip plating and in Example 3 which an Sn—Zn alloy layer isformed by applying alloy electroplating.

[0051] The evaluation results of the performances of ComparativeExamples are shown in Comparative Example 1-1 in the case of applyingthe thermal diffusion treatment after electroplating and then forming achromate film, in Comparative Example 1-2 in the case of not applyingchromate treatment and in Comparative Examples 2 and 3 in the cases of#100 tinplate and terneplate, respectively.

[0052] As seen in Examples, the case of employing the inorganic filmcontaining P+Mg is better than the case of employing the chromatetreatment in solder wettability after the accelerating treatment in aretort, shows a similar good performance in the rust-proof property, andshows more excellent performance than the #100 tinplate and theterneplate which are included in Comparative Examples. TABLE 1 Platinglayer Surface film Sn-Zn P + Mg (a) (b) (c) alloy Zn/Sn ratio depositedSolder Whisker- Incidence of Plating amount (in weight Treatment amountwett- Proof red rust method (g/m³) ratio) Condition (mg/m²) abilityproperty (area percent) Example 1-1 Electroplating + 10 0.1 (5) 10 ⊚ ◯ 2thermal diffusion treatment Example 1-2 Electroplating + 10 10 (5) 25 ◯◯ 1 thermal diffusion treatment Example 1-3 Electroplating + 15 0.01 (5)14 ⊚ ◯ 1 thermal diffusion treatment Example 1-4 Electroplating + 5 0.1(5) 17 ⊚ ◯ 3 thermal diffusion treatment Example 2-1 Electroplating + 351 (5) 20 ◯ ◯ 1 hot dip plating Example 2-2 Electroplating + 35 0.1 (5)11 ⊚ ◯ 2 hot dip plating Example 3 Alloy 10 0.1 (5) 14 ⊚ ◯ 1electroplating Comparative Electroplating + 10 0.05 (8) 2 mg/m² x ◯ 3example 1-1 thermal diffusion metallic treatment chromium conversionComparative Electroplating + 10 0.05 None — Δ ◯ 15 example 1-2 thermaldiffusion treatment Comparative Electroplating + 12 0 Dichromic 5 mg/m²Δ x 20 example 2 thermal diffusion acid, in metallic treatment electro-chromium (#100 tinplate) lysis conversion Comparative Hot dip plating 35— — None Δ ◯ 1 example 3 (terneplate)

INDUSTRIAL APPLICABILITY

[0053] As explained above, a surface treated steel sheet according tothe present invention has excellent performances in solder wettabilityafter a retort treatment and a rust-proof property and a whisker-proofproperty which are suitable for electronic components. The presentinvention makes it possible to supply a surface treated steel sheet withless environmental impact for electronic components.

1. A surface treated steel sheet with less environmental impact forelectronic components excellent in solder wettability, a rust-proofproperty and a whisker-proof property and having an Sn—Zn alloy layerwhich is formed by plating Sn and Zn on a steel sheet or a Ni platedsteel sheet and then applying thermal diffusion treatment, or by platingSn—Zn alloy on a steel sheet or a Ni plated steel sheet, characterizedin that the amount of said Sn—Zn alloy layer is not less than 3 g/m²,the Zn/Sn ratio (in weight ratio) is 0.01 to 10 and an inorganic filmmainly composed of magnesium phosphate is formed in the amount of 1 to100 mg/m² in terms of the amount of P+Mg on said Sn—Zn alloy layer.
 2. Asurface treated steel sheet with less environmental impact forelectronic components excellent in solder wettability, a rust-proofproperty and a whisker-proof property according to claim 1,characterized in that the Zn/Sn ratio (in weight ratio) of an Sn—Znalloy layer is 0.01 to 0.1.